Effect of zinc oxide coatings on the fatigue life of steel specimens

Initial fatigue tests on steel specimens with a layer of zinc oxide and glycerin – often applied for crack detection – indicate an influence of the coating on the fatigue life. Systematic fatigue tests on notched, unnotched, and welded specimens with and without zinc oxide have been performed to quantify the effect. Fatigue tests indicate an influence of the zinc oxide coating that becomes more significant with increasing dominance of the crack propagation stage. In welded stud specimens, the fatigue life increased by a factor of more than four compared to uncoated specimens. In coated notched round specimens with a stress concentration factor of K t ≈ 2, fatigue life is more than doubled relative to uncoated specimens. For unnotched round specimens, no significant effect was observed. Metallographic analysis revealed that parts of the zinc oxide coating were drawn into the crack. This indicates a potential mechanism where a viscous fluid penetrates the crack gap and aids load transfer. This reduces the stress intensity factor responsible for crack propagation, which leads to an increased fatigue life of coated notched components. These findings serve as a basis for further investigations of zinc oxide coatings on notched metallic components, supporting the mechanism.